1. Field of the Invention
The present invention relates to a magnetic tape medium on which recording and reproducing are performed in the longitudinal direction, and more particularly, relates to a magnetic tape medium having superior dimensional stability in the width direction and superior off-track resistance.
2. Description of the Related Art
In recent years, the amount of information has been explosively increasing because of the spread of the Internet, digital recording, and the like, and recording media for recording information as backup data have been developed toward higher capacity and higher recording density. A recording tape medium, which is wound and is placed in a cartridge, for recording backup data has been used in a commercially available linear recording system in which, in order to achieve a higher recording capacity, recording and reproducing are performed on a tape in the longitudinal direction thereof using a fixed head composed of many magnetoresistive heads. In order to further increase a recording capacity, various methods have also been attempted: short-wavelength recording is performed by decreasing the thickness of a magnetic recording layer, the track density in the tape width direction is increased, and the thickness of a tape is decreased so that the length per one reel is increased. In addition, for example, by disposing many heads or increasing a relative speed between a tape and a head, increase in transfer speed of data has also been performed.
Among the methods described above, when the short-wavelength recording is performed by decreasing the thickness of a magnetic recording layer, due to a small spacing between the magnetic recording layer and the head, problems may arise in that the output decreases, running durability degrades when the magnetic recording layer is allowed to run many times, and the like; hence, various types of research have been carried out. In addition, when the track density in the tape width direction is increased, due to the change in position of the tape in the width direction during high speed running and/or the change in dimension of the tape itself caused by environmental factors, a so-called off-track phenomenon may occur in which the track is not present at a predetermined tape position which is to be read by a magnetic head, or the position of the track is shifted. In particular, when a thin film medium is used, since running properties become unstable and the change in dimension of the tape itself caused by environment factors is increased, the problem described above becomes more serious. In order to avoid this off-track phenomenon, a method has been proposed in which a servo signal is recorded on a tape so that tracking is properly performed. As this servo signal recording method, for example, there is known a method in which a magnetic or an optical servo signal is recorded on a tape magnetic layer side or a back layer side.
As for the change in position of a tape in the width direction during operation, the change being one factor causing the off-track phenomenon in the tape width direction, a method has been proposed in which undulation of a tape edge is suppressed (for example, see Japanese Unexamined Patent Application Publication No. 2003-132526). In addition, as a method for suppressing another change, that is, the change in dimension of the tape itself which is caused by environment factors, for example, a method has been proposed in which the coefficients of thermal and hygroscopic expansion of a tape or a non-magnetic substrate are decreased (for example, see Japanese Unexamined Patent Application Publication No. 11-250449). However, when the track density in the width direction is relatively low, and a permissible off-track amount (=((recording track width)−(reproducing track width))/2, hereinafter referred to as “off-track margin”) is sufficiently large, practically, the problem is not serious in the actual environment, and the same thing can also be said when the change in position of the tape in the width direction during running is relatively large.